(a) Field of the Invention
Broadly speaking, this invention relates to electronic intruder-detection systems. More particularly, in a preferred embodiment, this invention relates to an electronic intruder-detection system which employs a transmission line as the sensing element.
(b) Discussion of the Prior Art
Most present-day intrusion-detection systems, including those that use line sensors, such as the balanced pressure system, should be classified as "point" sensor systems, i.e., systems which use independently powered and operating detectors having a limited range (in the order of several hundred feet) and zero range resolution. Typically, when surveillance over an extended boundary is required, many such sensors are employed side by side, each covering its respective segment and reporting alarm information to a central location. By the association of an alarm with the corresonding sensor the intruder is located, resolution being given by the spacing of the individual sensors.
For more efficient protection of a fixed installation perimeter, or surveillance over any other similarly defined narrow or irregular boundary, a "line" sensor is required, i.e., a sensor whose field of sensitivity is confined to a long, narrow corridor and which provides adequate range resolution. Such a system would comprise a suitable medium which guides energy with sufficiently low loss to cover as much as a mile of range, but whose field is also sufficiently "open" so that an intruder will cause a disturbance. Target location would then be accomplished by means of range gating, as commonly used in radar. This system would thus provide resolution in range (along the boundary) to avoid the need for separate electronics modules at every resolution element of coverage.
An intrusion detection system as above-described would have to satisfy the following criteria. First, the target requirements include the detection of not only walking and running intruders but also the stealthy, highly informed intruder who may approach the protected area crawling as slowly as 0.1 feet per second. Second, the intruder should trigger an automatic alarm to fix his location with a resolution suitable to the reaction which the operating procedures require. The desired resolution might, thus, vary from perhaps 100 feet to 1000 feet. Because intrusions will occur infrequently in such a surveillance system, but since an intrusion may prove very costly if successful, the detection probability for this type of target should be virtually 100%, while the false-alarm rate should be very low. For an electro-magnetic system, this will require exceptional moving clutter rejection. For an actual installation, this requirement may demand a certain amount of foliage control. Finally, the system should be of relatively low cost per unit distance (compared to present sensors) and feature ease of installation and maintenance, as well as simplicity of operation.
The very nature of the problem also imposes certain limitations on the system. For example, animal activity is a potential source of false-alarms. A small animal which moves very close to a sensor can easily produce the same magnitude of disturbance as a man at a greater distance. Furthermore, the small cross-section of a prone intruder and his extremely low speed present limitations to a system which must reject moving foliage clutter. Finally, operation under certain weather conditions, particularly rain, snow and wind may also present limiting factors.